If $400 is your ceiling, then you are pretty much limited to a good video card, and maybe a processor upgrade. I am not a big fan of sinking money into a system that has no future, and anything socket 775 falls into that category.
If it was me, I would first see how much headroom I have on that CPU. I have a lowly E5200 at 3.0 ghz on the wife's system, at stock voltage and cooling, and it runs like a top. I did have it to ~3.3, but really didn't see much improvement, and it tended to get a bit warm with stock cooling. Better safe than sorry, especially on a system that isn't mine. No reason that E5400 cannot get close to 3.2ghz without a sweat.
After that, I would put my money into a good video card, something that could be carried forward to my next build. I would be tempted to up my budget to around the $200 mark for a GTX560. I looked on the Egg, and they were close to the $200 mark after rebate.
With a CPU running at 3+ghz, and a new video card, you should be good to go on most new games. Some games are CPU crushers, flight sims, WOW, and GTA4 etc. If you can get a quadcore cheap, that's one thing, but spending money on a new CPU that is EOL just doesn't make sense. If the CPU is really limiting you, it is time to save up the cash, and build a Socket 1155 system, or see what AMD has to offer in the next couple months. I just quickly put together prices on a 2500K/P67/4gig mem and it came out to just under $400.
Edited by dpunisher, 28 April 2011 - 09:55 AM.
I am a retired Ford tech. Next to Fords, any computer is a piece of cake. (The cake, its not a lie)
3770K @4.5, Corsair H100, GTX780, 16gig Samsung, Obsidian 700 (yes there is a 700)